How do I connect a windows laptop to an iMac?

Hi there, in short I would like to connect my HP laptop to my iMac display (model A1418 100-240v) so I can this as a main monitor as the HP screen is pretty small.
The iMac has both thunderbolt and USB connections and the HP laptop has an HDMI connection.

I went to PC world and they said I need to buy a Mini DisplayPort to HDMI adaptor and an HDMI cable so I did. I have connected these and pressed command + F2 on the iMac keyboard (which it says I need to do on Google search), however NOTHING happens! Please could you advise on how I can sort this out so it works and I can drag windows etc onto the iMac and use this as a dual display. It would help me sooooo much if someone could help me please, many many thanks in advance. Please let me know if you need to know anything else.

Thanks for that, I did already see this, I'm not entirely sure which model the iMac is and typed in the serial number to try and find out, however, no luck. I am not that much of a 'techy' so finding it dificult to understand all of it. Do you think its possible the HDMI cable might not work? I bought this brand new the other day and have connected it using this. It looks as though it doesn't recognise there is even a second monitor when its connected...

On the iMac try cmd+f2 at the login screen and then on the HP laptop FN + F? to cycle through the display options of Laptop only, external only, both. You will need to check your HP info for the correct F key.